The present invention relates to multi-dimensional image reconstruction and, more particularly, but not exclusively to such image reconstruction based on a diffuse radioactive source or sources.
Radiological imaging is generally carried out on a living target, which of course means a mix of tissues in close proximity, if not actually overlapping. The general procedure is to feed the patient with one or more radioactive markers prior to the imaging process. The radioactive markers are taken up by the digestive system and pass into the bloodstream. From the bloodstream the marker passes into the different tissues at varying rates depending on the tissue type. Some tissues absorb markers faster than others and some tissues absorb certain markers faster than others. Furthermore certain tissues flush out the markers faster than others, and again the rate of flushing out may also depend on the kind of marker being used.
As a result, radioactive marking in fact creates a dynamic system in the body in which the relative darkness of a given tissue is related to a time factor. The radiologist knows that if he wants a good image of say the liver following application of a given marker then he should wait a certain number of hours from application of the marker before taking the image. Even so, the liver is not differentiated clearly from the other tissues.
Examples of radiopharmaceuticals include monoclonal antibodies or other agents, e.g., fibrinogen or fluorodeoxyglucose, tagged with a radioactive isotope, e.g., Technitium-99m, Gallium-67, Thallium-201, Indium-111, Iodine-123, Iodine-125 and Fluorine-18, which may be administered orally or intravenously. The radiopharmaceuticals are designed to concentrate in the area of a tumor, and the uptake of such radiopharmaceuticals in the active part of a tumor, or other pathologies such as an inflammation, is higher and more rapid than in the tissue that neighbors the tumor. Thereafter, a radiation-emission-measuring-probe, which may be configured for extracorporeal or intracorporeal use, is employed for locating the position of the active area. Another application is the detection of blood clots with radiopharmaceuticals such as ACUTECT from Nycomed Amersham for the detection of newly formed thrombosis in veins, or clots in arteries of the heart or brain, in an emergency or operating room. Yet other applications include radioimaging of myocardial infarct using agents such as radioactive anti-myosin antibodies, radioimaging specific cell types using radioactively tagged molecules (also known as molecular imaging), etc.
The usual preferred emission for such applications is that of gamma rays, which emission is in the energy range of approximately 11–511 KeV. Beta radiation and positrons may also be detected.
Radioactive-emission imaging is performed with a radioactive-emission-measuring detector, such as a room temperature, solid-state CdZnTe (CZT) detector, which is among the more promising that is currently available. It may be configured as a single-pixel or a multi-pixel detector, and may be obtained, for example, from eV Products, a division of II–VI Corporation, Saxonburg Pa., 16056, or from IMARAD IMAGING SYSTEMS LTD., of Rehovot, ISRAEL, 76124, www.imarad.com, or from another source. Alternatively, another solid-state detector such as CdTe, HgI, Si, Ge, or the like, or a combination of a scintillation detector (such as NaI(TI), LSO, GSO, CsI, CaF, or the like) and a photomultiplier, or another detector as known, may be used.
Considering the issue in greater detail, certain biological or chemical substances such as targeted peptides, monoclonal antibodies and others, are used for tagging specific living molecules for diagnostic purposes. Ideally, these antibodies are specific to the desired type of cells, based on adhering only to specific molecular structures in which the antigene matching the antibody is highly expressed. The use of imaging devices such as a nuclear gamma probe or a visual video probe can detect radiation emanating from taggants such as radionuclei or fluorescent dies that have been appended to the antibody before being delivered to the living body. An example is a cancerous cell of a prostate tumor on whose membrane there is an over expression of the Prostate Specific Membrane Antigen (PSMA). When a monoclonal antibody (Mab) such as Capromab Pendetide (commercially available as ProstaScint manufactured by Cytogen Corp.) is labeled with radioactive Indium (In 111) and is systemically delivered to the body, the Mab is carried by the blood stream and upon reaching the prostate tissue, adheres to the PSMA. The high energy radiation photons emitted by the radioactive Indium can be detected using a nuclear camera, indicating the presence and the specific location of the tumor.
Unfortunately, given the complexity of living organisms, in many instances the same antigen is also expressed in more than just the tissue under investigation. The antibody will thus also “paint” additional tissues such as infection areas, in addition to the tissue of interest. The radioactive readings taken from this additional tissue will be falsely interpreted as tumor areas, reducing the specificity of the test being performed.
The ‘Target to Background’ ratio that characterizes every such antibody for a given target cell type is one of the major issues that determine the ability to perform proper diagnosis, and guided procedures.
Since the uptake clearance of such a marker by the various tissues (target and background) varies over time, standard diagnosis protocols usually recommend taking an image at the time at which the ratio of Target emission vs. Background emission is the highest.
In an experimental system tried out by researchers, two markers were supplied to various patients and then images were taken at successive intervals for each of the markers. Certain features in the target areas showed up clearly in all images, other features were clear for all images of one marker but faded in and faded out for the other marker, and yet other features faded in and out for both markers but at different times. The researchers were able to use their knowledge of the behaviors of the two markers with different tissues in order to identify the features in the images.
The above system therefore relies on the knowledge of the researchers to put together information received from multiple images into an understanding of what the radio-imaging shows. In the general hospital environment it is not possible to guarantee that the necessary expertise is available, at least not for the amount of time that such a system would require.
There is thus a widely recognized need for, and it would be highly advantageous to have, a radiological imaging system devoid of the above limitations.